The present invention is related to ferroelectric memories, and more specifically, to a depolarization technique used for minimizing imprint in the ferroelectric memories.
In ferroelectric capacitors in ferroelectric memories, polarization is switched from one state to the other with an applied electric field. The desired behavior is that the polarization will switch easily from one state to the other, regardless of how long the first state has been stored or what its storage temperature profile has been. But in real-world parts, some capacitors will have a tendency to favor the state they have been storing for a long time. This is known as imprint as the state stored becomes imprinted onto the polarization orientation. If a capacitor becomes imprinted, it will be more difficult to write the new opposite state when desired. In extreme cases, the capacitor may not be able to switch to the opposite state.
High temperature excursions while a capacitor is polarized can accelerate imprint. The temperature range required for packaging is such an excursion that can accelerate imprint. Imprint caused by assembly can have a yield impact. Polarization is the driving force for imprint. By depolarizing the capacitors before packaging, imprint can be significantly reduced and yield and reliability improved.
FIGS. 1(a)-1(c), entitled “Polarization Configurations”, show three polarization configurations for a ferroelectric capacitor.
In FIG. 1(a), a Thermally Depolarized PZT Ferroelectric Capacitor (>400° C.) shows that the polarization vectors are randomly oriented. Thermal depolarization further means that the polarizations are randomly oriented and the net polarization is zero. It is the most effective and complete way to depolarize a ferroelectric capacitor. However, it requires high temperatures that might generate changes to the product and impact the performance of the circuits. Another limitation of thermal depolarization is that there is no selectivity and every ferroelectric capacitor is depolarized. In a more advanced ferroelectric device, some ferroelectric capacitors are used for storing engineering and configuration information, and such data should never be changed or lost.
In FIG. 1(b), a net polarization in a Polarized Ferroelectric Capacitor leads to imprint during the process that the device is packaged. The thermal budget of package assembly is equivalent to ˜185° C. for 4 hours. Although the parts are not powered up, a net polarization exists in the ferroelectric capacitors due to using the conventional testing process. The polarization vectors are always the same as the polarity of the last electric pulse to polarize the capacitor during wafer test.
In FIG. 1(c), an Electrically Depolarized Ferroelectric Capacitor shows a desired net zero polarization of the ferroelectric capacitor. Since the polarization is the driving force for imprint, zero net polarization or much reduced polarization will eliminate or reduce imprint during packaging. This invention teaches how to eliminate or reduce polarization by thermal depolarization and electrical depolarization.